Phased array receivers are used in various wireless communications systems to improve the reception of radio frequency (RF) signals. FIG. 1 is a drawing illustrating the principal components of a typical phased array receiver 100. The phased array receiver 100 includes a plurality of receive paths 102-1, 102-2, . . . , 102-n (where n is an integer greater than or equal to two), an RF combiner 104, and a downconverter 106. The plurality of receive paths 102-1, 102-2, . . . , 102-n includes antennas 108-1, 108-2, . . . , 108-n, low noise amplifiers (LNAs) 110-1, 110-2, . . . , 110-n, variable gain elements 112-1, 112-2, . . . , 112-n, and phase shifters 114-1, 114-2, . . . , 114-n. 
The amplitudes and phases of RF signals received by the antennas 108-1, 108-2, . . . , 108-n and amplified by the LNAs 110-1, 110-2, . . . , 110-n are controlled by the variable gain elements 112-1, 112-2, . . . , 112-n and phase shifters 114-1, 114-2, . . . , 114-n, respectively. Typically the amplitudes and phases are controlled in such a way that reception is reinforced in a desired direction and suppressed in undesired directions. Amplitude and phase adjusted RF signals in the plurality of receive paths 102-1, 102-2, . . . , 102-n are combined by the RF combiner 104, and then downconverted to intermediate frequency signals by the downconverter 106.
Successful operation of the phased array receiver 100 requires that the receive paths 102-1, 102-2, . . . , 102-n be precisely calibrated. When operating at RF, this requires that the physical characteristics of the transmission lines or cables used to connect the various RF elements in the plurality of receive paths 102-1, 102-2, . . . , 102-n be controlled with a high degree of mechanical precision. Unfortunately, this high degree of mechanical precision is both time consuming and very expensive.
Acceptable calibration and operational control of the phases of the received RF signals in and among the plurality of receive paths 102-1, 102-2, . . . , 102-n of the phased array receiver 100 also calls for phase shifters 114-1, 114-2, . . . , 114-n that are capable of controlling signal phases both accurately and with high resolution. Together, accuracy and high resolution afford the ability to maximize the phase alignment of the RF signals at the input of the RF combiner 104, thereby optimizing the reception capabilities of the receiver 100. Unfortunately, phase shifters that offer both accuracy and high resolution at RF frequencies, and which are also inexpensive to manufacture, are not readily available.
Generally, prior art phased array receivers employ one of two types of phase shifters. The first type of phase shifter 200, shown in FIG. 2A, includes a plurality of selectable transmission line sections 202-1, 202-2, 202-3, . . . , 202-n configured as delay elements. Typically, the selectable transmission line sections 202-1, 202-2, 202-3, . . . , 202-n are strip lines or microstrip lines formed in a monolithic microwave integrated circuit (MMIC). Junctions formed between adjacent transmission line sections 202-1, 202-2, 202-3, . . . , 202-n are selectably shunted to ground by selected operation of transistors 206-1, 206-2, . . . , 206n−1. Which of the transistors 206-1, 206-2, . . . , 206n−1 is ON and which is OFF is determined by a controller 208. An RF input signal that is launched from a circulator 204 and which encounters the first short circuit signal in its path (determined by which of the transistors 206-1, 206-2, . . . , 206n−1 is ON) is reflected back to the circulator 204, appearing as an RF output signal RFOUT. The phase difference between the phase of RFOUT and the phase of RFIN is, therefore, proportional to twice the sum of the lengths of the transmission line sections over which the RF signal traveled.
The phase shifter 200 in FIG. 2A can be made so that it is quite accurate. However, because there only a few discrete phase shift values available, the resolution to which the phase shifts can be controlled is quite low, particularly when the RF signals being shifted have very high frequencies. FIG. 2B is a drawing of a second type of phase shifter 200′ commonly used in phased array receivers, and which offers a higher resolution than the phase shifter 200 in FIG. 2A. The phase shifter 200′ comprises an in-phase mixer 220, a quadrature mixer 222, and a summer 224. The in-phase and quadrature mixers 220 and 222 are configured to mix an RF input signal RFIN with in-phase (I) and quadrature (Q) signals. Phase shifts to RFIN are introduced by varying the amplitudes of the I and Q signals. The resulting phase shifted signal RFOUT appears at the output of the summer 224.
Although the phase shifter 200′ in FIG. 2B can be controlled with greater resolution than the phase shifter 200 in FIG. 2A, it is not very accurate. In particular, when configured in multiple receive paths of a phased array receiver, gain variations among the phase shifters 200′ in the different paths, along with even small misalignments of the I and Q signals applied to the multiple phase shifters 200′, result in inaccuracies among the phases of the RF signals in the multiple receive paths 102-1, 102-2, . . . , 102-n. 
Considering the foregoing drawbacks and limitations of prior art phased array receiver approaches, it would be desirable to have phased array receivers and methods that provide the ability to control the phases of signals both accurately and with high resolution, and which also are not burdened by expensive and difficult calibration techniques requiring a high level of mechanical precision.